The photovoltaic effect, the capacity of some materials to absorb light and convert it to electricity, was known for more than a century before its potential as a practical source of energy was recognized. The importance of solar cells in space vehicles in recent years provided impetus to a flurry of investigations of various materials adaptable to the solar cells and to a variety of structures using these materials. From the standpoint of cost which, of course, includes both the cost of materials and efficiency of the solar cell to convert light into electricity, thin-film solar cells have become most attractive.
With an increasing number of photoactive materials and a variety of structures and methods of fabrication facing the designer of solar cells, the selection of the best combination becomes quite complex. Many factors infuencing efficiency are known and have been categorized by Barnett and Rothwarf in a paper "Thin-Film Solar Cells: A Unified Analysis of their Potential", in IEEE Transactions on Electron Devices, ED-27, No. 4, Apr. 1980, pages 615 to 630. In that paper, which describes a generic photovoltaic solar cell to which the present invention applies, major factors contributing to losses are discussed. Principal among those factors are optical losses, resulting in inefficiency in creation of carriers, and electrical losses, resulting from recombination of carriers in the semiconductor material and those occurring at interfaces of elements of the solar cell. It is to these losses that the present invention is primarily addressed.
Should actual solar cells be made in accordance with optimum materials and structures based on theoretical values, devices with efficiencies as great as 26 percent would result, substantially more efficient than the best of current production devices. That such high efficiencies are not routinely attainable, however, is partly a result of loss factors which are to a significant extent competitive: The longer the pathway of light through a photovoltaically active material (the absorber-generator), the greater the conversion of photons to carriers, but the greater the distance the carrier must travel to the collector-converter or to an electrically conductive pathway connected to the utilization circuit, the greater the recombination of carriers and loss of current generation capability.
Accordingly, structures providing a compromise between these competitive factors have been devised. Such structures employ thin films to minimize the distance a light-generated carrier must move in the absorber-generator layer to the converter-collector and thence to a conductor to thereby reduce the amount of recombination, and in combination, reflective means are provided to cause a multiplicity of reflections and passes of light through the absorber, increasing the distance light passes through and increasing light absorption, therefore current generation while decreasing the probability of recombination of carriers. The sheer number and continued appearance of new approaches with many variations of devices relying on thin films in combination with multiple reflections is indicative of the persistance of unsolved problems in this field.
A general method of achieving multiple reflections is to provide textured surfaces which cause reflections at a variety of angles back through the active layers. In the patent literature, there are disclosures on texturing by sandblasting (U.S. Pat. No. 3,487,223, issued to A. E. St. John, Dec. 30, 1969), by grooving the back of a transparent substrate (U.S. Pat. No. 3,973,994, issued to David Redfield, Aug. 10, 1976) and a variation of grooving, formation of a two-dimensional diffraction grating etched by electron beams on a substrate wherein the patterned surface is coated with silver or other conductive material (U.S. Pat. No. 4,493,942, issued to Ping Sheng, et al., Jan. 15, 1985).
The use of blocking layers, as described by Barnett in U.S. Pat. No. 4,251,286, to avoid undesired electrical contacts due to discontinuities in the active elements of solar cells and to prevent unwanted migration of contaminants into the photovoltaically active elements, inherently increased flexibility of design of solar cells. The provision of controlled electrical conductivity through insulating blocking layers is described in U.S. Pat. No. 3,988,167, issued to H. Kressler, et al., which discloses a solar cell having a semiconductor as a photovoltaically active element in which one surface has a non-continuous oxide layer thereon. The layer is non-continuous by virtue of openings through the layer arranged in a precise, preselected pattern to coincide with grid electrodes. The complexity of the fabrication procedure disclosed, involving formation of the oxide coating on what could be fragile silicon wafers, creation of the openings in a precise pattern by photolithography, vacuum deposition of conductive material (gold or chromium) in the openings, and removal of excess conductive material between openings by a second photolithographic step, however, appears hardly conducive to development of low-cost, rugged by efficient solar cells.
U.S. Pat. No. 4,571,448, issued Feb. 18, 1986, to Allen M. Barnett, incorporated herein by reference, discloses the use of barrier layers, in particular, the function of such as quarter wave reflectors to increase the efficiency of solar cells (col. 5, lines 40 to 59). Also disclosed therein is the use of liquid phase epitaxy in fabrication of solar celsolar cells.
While
While inventions in these disclosures and many others may have resulted in improvements within the scope of their objectives, there remain many specific areas where there is a need for additional improvements. Among these is a need for improvements in solar cell structures and a process for making it which can utilize relatively low cost materials which provide multiple reflections while minimizing internal and boundary recombinations. In this regard, silicon is an effective photovoltaic material in the very pure state with appropriate doping, but it has a low absorption and conversion of light to charge carriers, especially in the longer wavelength portion of the solar spectrum, so that long pathways in this material are required. Widespread use of silicon thin films in solar cells provided with means for internal reflection, however, has been inhibited to some extent by the difficulty finding economically attractive, reflective surfaces on barriers or substrates for silicon active-layer deposition in which surface and/or boundary recombination is sufficiently low so as to not offset any advantage in low recombination rates in the bulk of the thin film structure.